This invention relates to realignment of teeth, and more specifically pertains to the micro-regional force applications that tighten the control of the orthodontic movement of teeth in multiple directions.
The field of dentistry known as orthodontics involves applying forces to teeth in their current positions to create stresses in the periodontal ligament which connects the teeth to the supporting bone. These stresses stimulate the apposition and resorption of the supporting bone matrix allowing the teeth to be repositioned in the matrix to the position desired by the treating dentist. The direction, magnitude, range and relative positions on a tooth of the forces applied, in combination with the properties of an individual tooth to be moved, determine the actual versus desired movement of the tooth.
Currently orthodontic forces are applied to teeth with a variety of force generators including wires, brackets and bands bonded to the teeth, springs, elastomerics, and elastic repositioning appliances or tooth positioners sometimes in the form of a polymeric shell or a series of polymeric shells. With conventional braces, a force is generated when a flexible arch wire is distorted and secured into a slot in a bracket bonded to a tooth. As the wire seeks its original shape, a force is created. Because the force is applied a distance from the center of resistance, CRes, of a tooth, a moment is created that induces the tooth to rotate. To control the rotation, the wire would be rectangular or square in cross-section and the rectangular slot in the bracket fits tightly around the wire. A controlling couple arises as the flat portions of the wire engage the flat portions of the bracket. The sum of the moment of the couple and the moment of the force determined if and how the tooth will rotate with positive and negative being assigned to the directions of rotation. The analysis of these forces centers upon the CRes. The forces generated by prior art elastic repositioning appliances, polymeric shells or tooth positioners are less controlled due to the difficulty in creating precise couples as a result of the imprecise location, magnitude and direction of the forces they generate.
Orthodontic tooth movement forces generated by elastic repositioning appliances, polymeric shells or tooth positioners as applied in the prior art are described in U.S. Pat. Nos. 6,786,721; 6,783,360; 6,767,208; 6,722,880; 6,699,037; 6,685,469; 6,682,346; 6,629,840; 6,626,666; 6,582,227; 6,572,372; 6,554,611; 6,471,511; 6,454,565; 6,450,807; 6,406,292; 6,398,548; 6,318,994; 6,309,215; 6,299,440; 6,217,325; 5,975,893; 5,186,623; 5,059,118; 5,055,039; 5,035,613; 4,856,991; 4,798,534; 4,755,139. The preceding patents have the problem of controlling the tooth rotations created upon all axes generated by the unwanted moments induced by the imprecise forces applied to the teeth. These rotations and forces create unwanted side effects including deviating from the intended treatment path or plan. With early generations of these elastic repositioning appliances, movement was limited to one or two simple stages and the error introduced by rotation was of little importance to achieving the desired outcome.
Then U.S. Pat. Nos. 6,786,721; 6,783,360; 6,767,208; 6,722,880; 6,699,037; 6,685,469; 6,682,346; 6,629,840; 6,626,666; 6,582,227; 6,572,372; 6,554,611; 6,471,511; 6,454,565; 6,450,807; 6,406,292; 6,398,548; 6,318,994; 6,309,215; 6,299,440; 6,217,325; 5,975,893, all pertain to a system and method where the initial tooth position creates an initial digital data representation of the dentition, a final target tooth arrangement is created as a final digital representation, and intermediate transitional tooth arrangements are described digitally to plan an orthodontic treatment. The orthodontic treatment is established at the beginning of treatment and has many transitional tooth arrangements over an extended time period. A polymeric shell appliance is created to correspond to each transitional arrangement. Each appliance has a cavity to receive at least one tooth whose geometry corresponds to the next transitional tooth arrangement. The prior art has shortcomings explained below that the present invention overcomes.
In the prior art, the orthodontic force arises haphazardly by the poor fit of prior art appliances to the current arrangement of the teeth and with the location, magnitude, range and direction of the forces determined by the geometry of the next transitional stage regardless of the effectiveness of those forces. The present invention sets up effective forces and moments but not ideal or perfect forces and moments. Computer modeling of teeth, used to determine the final tooth arrangement from an orthodontic treatment path, excludes supporting structures such as root length, root shape, bone height, root surface area, and the position of forces applied to these structures.
When the appliance is installed, the poor fit of the appliance distorts its shape, which creates an orthodontic force as it tries to return to its shape. As the appliance approaches its undistorted geometry, the force dwindles because the magnitude of the force is a function of the degree of distortion. This decrease in orthodontic force can limit the intended movement, as the force drops below an effective therapeutic level before reaching the target transitional tooth position. This shortfall, as a result of inadequate force, though of little significance in a short sequence of appliances, may accumulate in a longer series of appliances thus deviating from the intended treatment path.
Controlling the rotational moment of the orthodontic force inherent in prior art flexible-positioning appliances that cause tooth rotation away from the intended treatment becomes more difficult by the cumulative effect of successive appliances, e.g. if each appliance creates a 2° unintended rotation, then over ten appliances the effect grows to 20°. A minor problem with one or two appliances now introduces a major error when applied over a greater number of appliances.
Referring to FIG. 3A, from the initial data of the tooth position, treatment is planned to advance the tooth bodily, with no rotation, to the final position. The long axis of the initial, and final tooth positions are preprogrammed to be parallel. If the tooth during treatment experiences unintended tipping, the long axis of the tooth will be inclined relative to the preprogrammed position. Because the existing polymeric shells were manufactured to fit a tooth with no rotation, and the tooth occupies a rotated position, a gap arises between the incisal edge of the tooth, and the polymeric shell creating the false appearance of intrusion. FIG. 3B shows where the teeth 1 have rotated around a different axis of rotation than planned in the appliances 2 creating the false appearance of intrusion with gaps 3 between the incisal edges of the teeth 1 and the appliances 2. This commonly occurs with a series of polymeric appliances, especially in the prior art. Accurate computer modeling of tooth movement from applied forces attempts to reduce false intrusion. However, common computer modeling in the absence of increased control of forces permits unwanted deviation from a narrow treatment path.
To prevent unwanted rotations and apparent intrusion the prior art has tightened the grip between the appliance and the tooth by adding protrusive attachments as later shown in FIG. 4 and varying the elastic modulus of the appliance; see U.S. Pat. Nos. 6,830,450; 6,705,863; 6,572,372; 6,524,101; 6,454,565; 6,309,215; and 5,176,517. This approach has seen marginal success because as a tooth rotates around a different axis of rotation than that of the polymeric appliance program, the grip of the attachment quickly loosens as it pulls out of the receptacle when the gingival portion of the tooth falls behind the rotating appliance. Once the attachment loosens unintended, unfavorable forces may arise and the options to recapture the tooth dwindle.
Controlling unwanted rotation with attachments also faces unfavorable physics. Referring to FIG. 5A, if the polymeric appliance apples 100 g of force on the attachment to move a tooth at 10 mm from the CRes, a moment of 1000 g*mm arises. To prevent tipping of a tooth, a couple must have equal magnitude and opposite direction to the moment. If the attachment is 4 mm long, and that all force applies at the ends, then an equal and opposite couple force is 1000 g*mm/4 mm or 250 g. This force exceeds that which the appliance can resist, the appliance displaces, and unwanted rotation occurs.
Rotation of a tooth around the vertical axis defies prediction in the prior art because the attachments commonly pull out of the polymeric shell. Effectively, the polymeric shell cannot distort in a mesial or distal direction. When using elastic repositioning appliances or tooth positioners in the form of polymeric shells, the predictable forces generated by the distortion of the appliance act buccally and lingually. FIG. 5B illustrates a poorly fit polymeric shell when placed over a tooth to be rotated, because the appliance cannot distort efficiently in a mesial or distal direction, the attachment will not fit into its concavity. The majority of force generated by the poorly fit appliance causes no orthodontic movement in the intended direction. The force generated in a therapeutic direction has no couple force to produce effective rotation and so the tooth falls behind the treatment plan.
In consequence, the above described deviations from the intended treatment path may cause the remaining appliances to have marginal effect or become unusable. Modifications to the prior art appliances is extremely limited. Reconfiguration of the appliance sequence to remedy the new, though unintended, tooth position calls for new impressions of the patient's teeth and expensive and lengthy rework of the appliance sequence.